Myomaker and Myomerger Work Independently to Control Distinct Steps of Membrane Remodeling during Myoblast Fusion. Thousands of mononucleated muscle cells fuse to generate multinucleated myofibers of mammalian skeletal muscle (1). Genetic screenings have identified two muscle-specific proteins essential for mammalian myogenesis: myomaker (Tmem8c) (2) and myomerger (Gm7325) (3-5). In our recent study (6) (collaboration with Dr. Douglas Millay, Cincinnati Childrens Hospital Medical Center), we explored functional interplay between contributions of these two proteins in myoblast fusion. We uncovered a fusion mechanism where myomaker and myomerger perform separate functions: Myomaker controls early fusion stage of hemifusion and Myomerger drives the transition from hemifusion to opening and expansion of a fusion pore. Myomaker-Myomerger physical interaction is not required for function. Myomaker does not need Myomerger to mediate hemifusion. Ectodomain of plasma membrane-anchored Myomerger acting from outside the cell drives fusion completion in a heterologous system independent of Myomaker, and, thus, Myomerger does not need Myomaker to drive fusion completion. Collectively, our data identify a novel step-wise cell fusion mechanism in myoblasts where different proteins are delegated to perform unique membrane functions essential for the coalescence of myoblast membranes. Interactions with muscle cells boost fusion, stemness and drug resistance of prostate cancer cells. Mechanisms by which interactions between cancer cells and non-malignant cells within the tumor microenvironment influence cancer progression and metastasis remain to be understood. Prostate gland is surrounded by smooth muscle of the prostate stroma and the striated muscle fibers of the rhabdosphincter. While both smooth and skeletal muscle cells are known to produce and secrete many signaling molecules, the effects, if any, of the muscle cells on the development and progression of primary prostate tumors are yet unexplored. In our recent study, we modeled these interactions in in vitro co-cultures (7). We found that interactions with primary smooth or skeletal muscle cells increase clonogenic potential and drug resistance of the primary prostate cancer cells. These interactions also expanded the fraction of the cancer cells expressing a stem cell marker CD133. We dissected the pathway of these muscle cell-induced changes in the properties of the cancer cells into several distinct steps. First, the interactions of the cancer cells with muscle cells upregulate expression and secretion of IL-4 and IL-13. Second, these cytokines upregulate expression of syncytin 1 and annexin A5. Third, upregulation of these proteins promotes fusion of the cancer cells. Suppressing either the cytokines activity or cell fusion blocks the increases in the stemness and drug resistance of the cancer cells co-cultured with muscle cells. On the other hand, the effects of the muscle cells can be reconstituted by either treating cancer cells with recombinant IL-4 and IL-13 or by fusing the cancer cells with heterologously expressed syncytin 1. Our work is the first report showing that interactions with primary smooth and skeletal muscle cells promote changes in the properties of primary prostate cancer cells that are consistent with their malignant progression. Furthermore, our study identifies three required and sufficient steps in the underlying pathway and thus provides definitive mechanistic insights into discovered phenomenon. The dissected pathway of microenvironment-driven changes in the properties of cancer cells represents a novel paradigm of cancer progression and may present new therapeutic targets. References: 1. Brukman NG, Uygur B, Podbilewicz B, Chernomordik LV. How cells fuse. J Cell Biol 2019;218:1436-51 2. Millay DP, O'Rourke JR, Sutherland LB, Bezprozvannaya S, Shelton JM, Bassel-Duby R, et al. Myomaker is a membrane activator of myoblast fusion and muscle formation. Nature 2013;499:301-5 3. Quinn ME, Goh Q, Kurosaka M, Gamage DG, Petrany MJ, Prasad V, et al. Myomerger induces fusion of non-fusogenic cells and is required for skeletal muscle development. Nat Commun 2017;8:15665 4. Bi P, Ramirez-Martinez A, Li H, Cannavino J, McAnally JR, Shelton JM, et al. Control of muscle formation by the fusogenic micropeptide myomixer. Science 2017;356:323-7 5. Zhang Q, Vashisht AA, O'Rourke J, Corbel SY, Moran R, Romero A, et al. The microprotein Minion controls cell fusion and muscle formation. Nat Commun 2017;8:15664 6. Leikina E, Gamage DG, Prasad V, Goykhberg J, Crowe M, Diao J, et al. Myomaker and Myomerger Work Independently to Control Distinct Steps of Membrane Remodeling during Myoblast Fusion. Dev Cell 2018;46:767-80 e7 7. Uygur B, Leikina E, Melikov K, Villasmil R, Verma SK, Vary CPH, et al. Interactions with Muscle Cells Boost Fusion, Stemness, and Drug Resistance of Prostate Cancer Cells. Mol Cancer Res 2019;17:806-20